Many sensors in technical applications generate charges that are output as sensor signals. Relatively small currents, for instance, can occur. For this reason it is usual for the charges to be integrated and converted into voltages. The photodiodes used in the sensor field of a digital X-ray machine are an example of this type of sensor.
In order to achieve adequate image quality in digital X-ray images, it is desirable for a circuit used for charge integration to exhibit a linear integration function along with low noise. In addition, a circuit of this type should be appropriate for the desired image or frame rates, and should consume low current, in order to avoid a rise in temperature and the associated effects this might have on temperature-dependent components.
Capacitative elements having low capacitances are frequently used for integration in order to reach higher voltages. Switches are also used to control the integration procedure. These often exhibit voltage-dependent parasitic capacitances, arising for instance from the depletion layer capacitance present when semiconductor switches are used. The parasitic capacitances can affect the accuracy of the integration. In addition, the sensor that generates the charge can also exhibit parasitic capacitances, again possibly arising because of the switches used, or in the form of conductor capacitances that can falsify the result of the integration result, depending on the noise in the circuit.
An attempt is made in conventional integration circuits, for instance, to compensate for the parasitic capacitances through elements with an opposing capacitative behaviour. Since, however, elements with entirely complementary capacitative behaviour are not generally available, residual non-linearities are still found. It is also difficult to compensate for any non-linearities that occur using subsequent digital processing, as the parasitic capacitances involved in each case vary as a consequence of process variations.